In audio systems, electrical cables are used to carry signals from one component to another, such as from a CD player to an audio preamplifier, or from a preamplifier to a power amplifier. Such cables are usually shielded in order to prevent a central conductor, conducting the audio signal, within the cable from picking up outside electrical signals, such as hum from power wiring and other interference. Shielding almost always consists of placing the central conductor within a grounded, highly conductive metal tube such as formed by woven copper strands, aluminized mylar tape, or the like. Sometimes a separate ground lead is placed within the shield and insulated from both the shield and the central conductor, or, alternatively, the shield itself may be used as the ground lead.
Similarly, the circuitry within audio equipment (e.g., CD players, preamplifiers, amplifiers, tape players, etc.) is sometimes shielded by means of a grounded metal box to block outside electrical signals from interfering with the low level audio signals being generated within the audio equipment.
All these above-mentioned shielding methods are based on the same principle that the wire or system to be shielded is placed within an highly conductive enclosure, usually made of metal. Such a shield is sometimes called a Faraday cage. Faraday cages are employed because Maxwell's equations (which govern electromagnetic phenomena) teach that there can be no electrostatic field penetration from the outside to the inside of a perfectly conducting, closed surface.
Although the above methods of shielding are effective in preventing externally generated electric fields from penetrating into the central conductor of a cable or into the enclosed audio circuitry, the above methods of shielding degrade the quality of the audio signals conducted in the audio cables or generated by the audio circuitry. Such degradations are audible to a well-trained ear, as confirmed by listening tests we have conducted; however, these degradations are subtle and are typically not detectable by standard oscilloscope traces, nor are they apparent on the usual distortion measuring equipment.
In the case of shielded cables, we believe that the cause of such degradation stems from the fact that the highly conductive shield intersects magnetic field lines generated by the current carried by the central conducting wire. These intersected field lines, in turn, induce eddy currents in the shield, which cause fluctuating voltage gradients (or electric fields) along the shield. These fluctuating voltage gradients are capacitively coupled to the central conductor and thus degrade the audio signal carried on the central conductor.
Similarly, the frequently used, highly conductive shielding cabinet surrounding an audio circuit may affect the sound quality of the audio circuit. The magnetic fields generated by currents flowing through the output leads of amplifier circuits within the cabinet may induce small eddy currents in the conducting walls of the cabinet. Such eddy currents produce fluctuating electric fields which, in turn, induce small currents in the input leads of amplifiers within the cabinet. The small induced currents in the input leads will be greatly amplified and thus may produce noticeable effects at the outputs of the amplifiers.
Thus, what is needed is an audio cable and improved type of shielding for audio circuits which do not suffer from the above-identified drawbacks with typical shielded cables and audio equipment cabinets.